FCC technology has undergone continuous improvement and remains the predominant source of gasoline production in many refineries. This gasoline, as well as lighter products, is formed as the result of cracking heavier, higher molecular weight, less valuable hydrocarbon feed stocks such as gas oil.
In its most general form, the FCC process comprises a reactor that is closely coupled with a regenerator, followed by downstream hydrocarbon product separation. Hydrocarbon feed contacts catalyst in the reactor to crack the hydrocarbons down to smaller molecular weight products. During this process, coke tends to accumulate on the catalyst. Coke must be burned off of the catalyst in a regenerator.
When a catalyst is exposed to oxygenates, such as methanol, to promote a reaction to olefins in a methanol to olefins process (MTO), carbonaceous material is generated and deposited on the catalyst. Accumulation of coke deposits interferes with the catalyst's ability to promote the MTO reaction. As the amount of coke deposit increases, the catalyst loses activity and less of the feedstock is converted to the desired olefin product. The step of regeneration removes the coke from the catalyst by combustion with oxygen, restoring the catalytic activity of the catalyst. The regenerated catalyst may then be exposed again to oxygenates to promote the conversion to olefins.
Conventional regenerators typically include a vessel having a spent catalyst inlet, a regenerated catalyst outlet and a combustion gas distributor for supplying air or other oxygen containing gas to the bed of catalyst that resides in the vessel. Cyclone separators remove catalyst entrained in the flue gas before the flue gas exits the regenerator vessel.
There are several types of catalyst regenerators in use today. The conventional bubbling bed regenerator typically has just one chamber in which air is bubbled through a dense catalyst bed. Spent catalyst is added and regenerated catalyst is withdrawn from the same dense catalyst bed. Relatively little catalyst is entrained in the combustion gas exiting the dense bed.
Two-stage bubbling beds and combustor regenerators have two chambers. In a two-stage bubbling bed regenerator, spent catalyst is added to a dense bed in a first, upper chamber stage and is partially regenerated with air. The partially regenerated catalyst is transported to a dense bed in a second, lower chamber stage and completely regenerated with air. The completely regenerated catalyst is withdrawn from the second chamber.
Complete catalyst regeneration can be performed in a dilute phase, fast-fluidized, combustion regenerator. Spent catalyst is added to a lower chamber and is transported upwardly by air under fast fluidized flow conditions while completely regenerating the catalyst. The regenerated catalyst is separated from the flue gas by a primary separator upon entering into an upper chamber in which regenerated catalyst and flue gas are disengaged from each other.
Catalyst coolers have been used to cool regenerated catalyst and permit the regenerator and the reactor to operate under independent conditions. In catalyst coolers, hot regenerated catalyst is cooled by indirect heat exchange with water which vaporizes to steam in nested cooler tubes. The steam is removed from the catalyst cooler for other uses; whereas, the cooled catalyst is returned to the regenerator.
Shut downs of a regenerator are costly due to the fact that product is not being made during shut down. Hence, shut downs should be minimized to maximize profitability.
If catalyst cooler operation is interrupted, the catalyst bed in the cooler must be re-fluidized. Improved ways of designing and operating catalyst coolers are sought.
Catalyst coolers are necessary to remove heat from the regenerator which is hot from burning hydrocarbon deposits from coked catalyst. As FCC units are called to process heavier feeds and MTO units are called to process more reactants, greater quantities of heat will need to be removed from regenerators. More efficient catalyst cooling equipment and processes are needed.